The present invention relates to a curable composition, and more particularly to a curable composition having improved storage stability.
A widely known composition, which upon exposure to atmospheric moisture is curable to form a rubbery substance, comprises an organic polymer having a cross-linkable group containing a silicon atom to which a hydrolyzable group is bonded (hereafter referred to as "hydrolyzable silicon group") as shown, for instance, in U.S. Pat. No. 3,971,751. The curable composition is suitable for use in sealants and other similar materials.
It is known that the curable composition may be used in the form of a two-package system in which a main component and a curing agent are separately prepared and maintained, and are then admixed shortly before use. Also known is a one-package system in which a main component and a curing agent are previously admixed and the mixture is stored in an airtight container in an anhydrous state. Upon application, the mixture is cured by moisture in the air. The one-package system is considered to possess superior workability to that of the two-package system. However, in some instances, the one-package cures even when it is in an airtight container, with a consequent decrease in the workability of the composition. Thus, there is a need in the art for a curing composition possessing excellent storage stability.
Poor storage stability of the known curable composition results from moisture in an inorganic filler containing a large amount of water, such as calcium carbonate, talc or silicon dioxide. The fillers are admixed with the curable composition to impart rubber-like properties to the cured product of the composition or to lower the cost of the composition. During storage, the viscosity of the composition increases, or the composition solidifies as a result of cross-linking of the hydrolyzable silicon groups in the organic polymer through hydrolysis and a silanol condensation reaction.
Although several methods are known which improve storage stability of the composition in the one-package system, each suffers from certain disadvantages. Methanol may be added to the curable composition and an azeotropic dehydration performed to remove the moisture, but the operation is complicated. Alternatively, silanes or siloxanes having a hydrolyzable silicon group may be added to the curable composition, but the modulus of elasticity of the cured product is too great. Additionally, an alkyl ester of orthoformic acid (such as methyl orthoformate) may be added to the curable composition, but the curable composition still suffers from insufficient storage stability. Further, storage stability may be improved by drying the inorganic fillers in vacuo, but a special vacuum dryer is required and consequently high equipment costs are incurred.
An object of the present invention is to provide a one-package curable composition possessing excellent storage stability by adding a filler other than an inorganic filler to the composition.
This and other objects of the present invention will become apparent from the following description.